Mesoscopic effects in ferromagnets could be different from mesoscopic effects in normal metals. While normal metals with a short mean-free-path do not exhibit classical magnetoresistance, weakly disordered ferromagnets with a similar mean-free-path display magnetoresistance including domain wall resistance (DWR) and anisotropic magnetoresistance (AMR). Magnetoresistance could lead to novel mesoscopic effects because the wave function phase depends on the scattering potential. In this thesis, we present our measurements of mesoscopic resistance fluctuations in cobalt nanoparticles and study how the fluctuations with bias voltage, bias fingerprints, respond to magnetization-reversal processes. The resistance has been found to be very sensitive to the magnetic state of the sample. In particular, we observe significant wave-function phase shifts generated by domain walls, and it is explained by mistracking effect, where electron spins lag in orientation with respect to the moments inside the domain wall. Short dephasing length and dephasing time are found in our Co nanoparticles, which we attribute to the strong magnetocrystalline anisotropy.

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This thesis presents an experimental study of the superconducting proximity effect in sub-micrometer sized ferromagnetic discs. Such discs belong to a class of mesoscopic ferromagnets intermediate between microscopic magnets with dimensions below about 10nm that behave as single giant spins and macroscopic structures that are larger than approximately 1 micrometer where domains are formed to minimise stray fields. The magnetic structure of mesoscopic magnets is strongly dependent on their geometric shape, allowing for purposeful engineering of magnetic structures using modern lithographic techniques. The ground magnetic state of mesoscopic ferromagnetic discs is the magnetic vortex where unusual time-asymmetric triplet superconductivity is predicted to exist and survive up to the non-magnetic coherence length, that is orders in magnitude larger than the ferromagnetic singlet coherence length. Magnetic Force Microscopy (MFM) was used to directly study the magnetic structure of the discs. To detect the proximity effect in the vortices, Andreev interferometers were used with normal parts replaced with mesoscopic ferromagnetic discs in the magnetic vortex state. The samples were fabricated using electron-beam lithography and a modified shadow evaporation technique developed within this project, allowing the whole structure to be made with highly precise alignment, without breaking vacuum and avoiding redundant ferromagnetic elements disturbing the magnetic vortices. Observations were made of superconducting phase periodic oscillations in the conductance of the Andreev interferometers. Such oscillations provide unambiguous evidence of phase coherent electron transport through the ferromagnetic vortex. Finally, further experiments are discussed that would provide a more detailed understanding of the long range proximity effect in SFS junctions.

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